PCB Technical - How to obtain the ideal interface organization in SMT processing

How to obtain the ideal interface organization in SMT processing
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We hope to obtain well-strengthened eutectic particles and solid solution structure through brazing. We hope to have a thin and flat bonding layer (0.5 ~ 4um) at the interface to minimize the appearance of composite layers in the brazed joint. Lead-free soldering hopes to get a solder structure with less isolation.

There are many conditions for obtaining the ideal interface organization, such as:

1. The mutual solubility of the metal component of the brazing filler metal and the base metal is good;

2. The surface of solder and base metal is clean, free of oxide layer and other impurities;

3. The effect of excellent surface active substances (flux);

4. Ambient atmosphere, such as nitrogen or vacuum protection welding;

5. Suitable temperature and time (ideal temperature curve);

6. The interface of the reaction layer can be kept flat, such as the expansion coefficient of the PCB material is small, and the PCB transmission system is stable.

Lead-free soldering temperature is high. In particular, the expansion coefficient of the PCB material in the z-axis direction is relatively small. It can maintain a plane interface reaction layer, otherwise, in the case of isolation, if the deformation pressure of the PCB, it is easy to cause the solder joints to be distorted or even the pad to fall off. In the conditions listed above, under other conditions unchanged, the main factors affecting the thickness of the bonding layer (brazed wire) and the composition and ratio of the intermetallic compound compound are temperature and time. If the temperature is too low, the bonding layer cannot be formed or the bonding layer is too thin; if the temperature is too high and the time is too long, the composite layer will become thicker, so it is very important to set the temperature curve correctly.
In the previous section, we analyzed the setting of the reflow soldering temperature curve. At the SMT chip processing plant, we did some analysis on the impact of brazing and the formation of excellent solder joints, because many PCBA considerations are double-sided mounting. This requires a second oven, causing many solder joints to be baked at high temperatures many times. How to obtain the ideal interface structure under the condition of repeated heating is one of the problems that the SMT chip factory must work hard to solve. At present, the country has higher and higher requirements for environmental protection and greater efforts in link governance. This is a challenge but also an opportunity for PCB factories. If PCB factories are determined to solve the problem of environmental pollution, then FPC flexible circuit board products can be at the forefront of the market, and PCB factories can get opportunities for further development.